Eeek, I see oil in the coolant reservoir of my W12 Touareg.

As far as I can tell, engine temps were stable and normal prior to this sudden occurrence. Based on this, my top suspect is not the head gasket, but the oil-cooler, which is a coolant/oil heat exchanger.

This is not a cheap part, and I expect the dealer to charge a handsome fee for labor, so I'd like to be sure that it is indeed the oil cooler and not something more invasive like a head gasket (shudder).

Here is a diagram of the oil circulation system on the W12

W12 Cooling System


Is there a way for me to diagnose the leak, preferably with the oil cooler in situ?

(I feel like I'm clutching at straws with the "in situ" part of the request)

A lot of YouTubers like to submerge the cooler in water and use compressed air to look for bubbles, but this obviously involves unbolting the oil cooler from its home in the engine bay.

If extraction is the only option, I can understand that - just that I'm not looking forward to the ordeal.


3 Answers 3


Another possibility as an alternative to @myself's idea is to connect pipes to the oil cooler and then connect them to a vacuum pump. If you pull a small amount of vacuum and it holds the vacuum overnight then the cooler is probably OK. This will avoid pushing water into the engine oil. A vacuum pump with a pressure gauge like one used for air conditioning evacuation would be ideal.

  • I like that idea, the advantage is that the test indicates if the leak would be a "one-way-leak". A drawback is that you need a perfect seal on the vacuum, even the slightest leak would provoke a false indication
    – Martin
    Aug 4, 2016 at 9:13

If those are coolant pipes on the side of it, you could join them together so that they bypass the cooler. If the leak stops you know that the cooler was causing the problem. You may also find that oil comes out of the coolant fittings which will also indicate the fault is with the cooler.

  • Great suggestion with the bypass, except that I wouldn't be able to keep the engine running for an extended length of time. This opens up a few possibilities for sure.
    – Zaid
    Aug 3, 2016 at 20:45
  • @zaid Do you think the oil cooler will be necessary for low throttle driving?
    – HandyHowie
    Aug 3, 2016 at 21:11
  • The sump's supposed to contain 13 L of oil so I don't see it overheating that quickly
    – Zaid
    Aug 3, 2016 at 21:11
  • Install an oil temp gauge, no telling why the engineers specified an oil cooler, possibly for towing conditions. I removed it on my 91 gmc 2500 as I do not tow anything, seems to be ok in my situation.
    – Moab
    Aug 3, 2016 at 21:12
  • @Moab: The oil cooler does not only cool the oil. Because coolant heats up faster than oil on startup it also provides a fast warmup of the engine oil.
    – Martin
    Aug 4, 2016 at 7:47


  • Remove the coolant pipes.
  • Connect two long, transparent, pressure resistant hoses.
  • Place the two hoses upward, fill them with (colored?) water and bleed the entire assembly using an external water pump or hose.
  • Mark the water level on the tubes
  • Figure out the designed cooling system pressure. Now put those two tubes under pressure (compressed air)
  • Let it stay overnight

If the water level has changed there must be a leak.

Perhaps you must adjust it to match your situation, but I am sure you get the idea. However, there is a possibility for a false negative: When the leak only opens when the oil/water is hot.

  • Good idea. The only problem here is that if the oil cooler is at fault you will be forcing water into the engine oil.
    – HandyHowie
    Aug 4, 2016 at 8:57
  • Yeah, true. However you can't exclude a present water contamination of the engine oil. So a oil-change is always necessary. He can also use engine oil instead of water, the coolant circuit atm. is definitely contaminated so he has to flush it.
    – Martin
    Aug 4, 2016 at 9:02

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